This invention relates in general to cast vehicle wheels and in particular to a method and an apparatus for casting such wheels.
Cast wheels formed from light weight metal alloys are replacing steel wheels on an increasing number of vehicles. Such cast wheels provide both a reduction in weight from steel wheels and an attractive appearance. Cast wheels are cast by pouring a molten metal, typically an alloy of a light weight metal, such as aluminum, magnesium or titanium, into a wheel mold assembly to form a wheel casting.
Referring to the drawings, there is shown in FIG. 1 a sectional view of a typical wheel casting, indicated generally at 10, formed in accordance with the prior art. For clarity, section lines have been omitted from FIG. 1. Additionally, in the following discussion of the wheel casting, "inner" refers to a portion of the wheel oriented towards a vehicle (not shown) when the wheel is mounted thereon while "outer" refers to a portion of the wheel oriented away from the vehicle.
The wheel casting 10 includes a relatively thin annular rim portion 11 which carries a vehicle tire (not shown). The rim portion 11 has a recessed center well 12 formed between inner and outer bead seats 13 and 14. An inner flange 15 is formed on the inner edge of the inner bead seat 13 and extends radially outward. An annular rim riser 16 extends axially upward in FIG. 1 from the inner flange 15. An outer flange 17 is formed on the outer edge of the outer bead seat 14 and extends radially outward. The flanges 15 and 17 function to retain a vehicle tire upon the finished wheel.
A wheel disc 18 extends across the outer end of the casting 10. The junction of the wheel disc 18 to the rim portion 11 forms a sidewall 14A which is accordingly thicker than either the wheel disc 18 or the rim portion 11. The wheel disc 18 typically includes a plurality of openings (not shown) formed therethrough which allow a flow of cooling air to the vehicle brakes while reducing the wheel weight. A wheel hub 19 is formed in the center of the wheel disc 18. The hub 19 includes a central aperture which receives an axle end (not shown) and a plurality of bolt holes (not shown) for attaching the finished wheel to a vehicle. A generally cylindrical hub riser 20 extends upward in FIG. 1 from the inner end of the wheel hub 19.
Once the wheel casting 10 has cooled sufficiently, it is removed from the mold and machined to final shape. The wheel also can be subjected to solution heat treatment and aging. The outer surface of the wheel disc is often highly polished and can include decorative painted portions.
The manner in which the molten metal forming the wheel casting 10 cools and the structure of the mold in which the wheel is cast affect the physical characteristics of the wheel. For the following discussion, portions of the wheel section shown in FIG. 1 are labeled with letters. The portion of the wheel 10 included in the inner flange 15 is labeled with the letter "A" and the narrowest section of the center well 12 is labeled with the letter "B". Similarly, the sidewall 14A is labeled with the letter "C" and the narrowest section of the wheel disc 18 is labeled with the letter "D". The wheel hub 19 is labeled with the letter "E".
The molten metal forming the wheel casting 10 solidifies first in the thinnest portions of the casting, since these portions cool the fastest. Thus, solidification of the molten metal typically begins in the shaded portions of the center well 12 and wheel disc 18 shown in FIG. 1 and labeled B and D. The solidification then proceeds towards the thicker portions of the wheel casting 10 as illustrated with small direction arrows on FIG. 1. As shown in FIG. 1, the solidification proceeds in two axial directions simultaneously through the rim portion 11, from B inwardly towards A, and from B outwardly towards C. Similarly, the solidification of the wheel disc 18 proceeds in two radial directions simultaneously from D outwardly towards C and inwardly towards E. Finally, solidification begins at the outboard flange 17 and proceeds towards C.
The cooling rate and thereby the solidification of the molten metal can be further affected by the particular mold geometry to the extent that it is not possible to identify solidification directions.
As the molten metal solidifies, a crystalline structure consisting of individual metal grains is formed in the wheel casting 10. The individual grains vary in size within the casting 10. Generally, the portion of the casting 10 that solidifies first has a smaller grain size and the grain size increases proportionally with the length of time required for cooling. The average grain size for the prior art wheel casting 10 as a function of position within the casting 10 is graphically illustrated in FIG. 2 by the line 30. The letters shown along the horizontal axis in FIG. 2 correspond to the letter labels shown in FIG. 1 and indicate the relative position on the wheel casting 10. For the typical wheel casting 10 illustrated in FIG. 2, the grain sizes vary over a wide range, from 44 to 106 microns (um). The minimum grain size illustrated in FIG. 2 corresponds to the portions B and D of the wheel casting 10 which solidify first. The larger grain sizes indicated for portions A and E, which solidify last, result in an increased porosity in these portions of the wheel casting 10. Large grain size causes a low structural casting strength and the high porosity can allow escape of the pressurized air contained in a tire mounted upon the finished wheel. The irregular shape of the line 30 indicates that solidification occurred in multiple directions in the rim portion 11 and the wheel disc 18.
The crystalline structure of the wheel casting 10 is related to the mold used to cast the wheel. Cast wheels are typically formed by gravity feeding or pressure injecting molten metal into a mold cavity formed in a multi-piece steel wheel mold assembly. A simplified sectional view of a prior art multi-piece mold assembly 40 for casting the wheel casting 10 is shown in FIG. 3. The individual pieces of the mold assembly 40 are typically formed from cast iron or high carbon steel. The mold assembly 40 includes a base member 41 which supports the other pieces of the mold assembly 40. Two or more retractable side members 42A and 42B are carried by the base member 41. A removable cup-shaped top member 43 having a cylindrical center portion 43A is disposed within the mold side members 42A and 42B.
The mold members 41, 42A, 42B and 43, upon assembly, define a main mold cavity 44 wherein the wheel casting 10 is cast. The main cavity 44 includes an annular rim cavity 45 for casting the wheel rim portion 11 and a disc cavity 46 for casting the wheel disc 18. An annular sidewall cavity 47 for casting the wheel sidewall 14A joins the rim cavity 45 to the disc cavity 46. The base member 41 defines the outer surface 46A of the disc cavity 46. Similarly, the side members 42A and 42B define outer surfaces 45A and 47A of the rim and sidewall cavities 45 and 47. The top member 43 defines inner surfaces 45B and 47B of the rim and sidewall cavities 45 and 47 and an inner surface 46B of the disc cavity 46. The side members 42A and 42B and the top member 43 further define an annular rim riser cavity 48 formed adjacent to the rim cavity 45. Similarly, a cylindrical hub riser cavity 49 is formed in the top member center portion 43A adjacent to the inner center of the disc cavity 46.
To cast a wheel casting 10, molten metal is fed by a conventional method, such as gravity or under pressure, into the main cavity 44 through a sprue (not shown). The molten metal flows into the disc, sidewall and rim cavities 46, 47 and 45, and then fills the rim and hub riser cavities 48 and 49. Molten metal flows from the riser cavities 48 and 49 into the main cavity 44 to fill any voids formed as the metal in the main cavity 44 cools and contracts.
To facilitate the flow of molten metal contained in the riser cavities 48 and 49 into the main cavity 44 as needed, the rim and disc cavities 45 and 46 are purposely made wide. The wide rim and disc cavities 45 and 46 join to form a wide sidewall cavity 47. Thus, the resulting wheel casting 10 has a relatively thick rim portion 11, sidewall 14A and wheel disc 18. The thick rim portion 11 and wheel disc 18 require a long cooling time for the molten metal forming them to solidify. Accordingly, the riser cavities 48 and 49 are made large to hold sufficient metal to assure that a portion of the metal within the risers remains molten until needed.